The high temperatures in hot gas channels and other hot gas spaces mean that it is necessary for the internal wall of a hot gas channel to be configured with the highest level of temperature-resistance possible. Materials with a high level of heat resistance, such as ceramic materials, are suitable for this purpose. But ceramic materials have the disadvantage that they are both very brittle and they also have unfavorable thermal and temperature conducting characteristics. Metal alloys with a high level of heat resistance and an iron, chromium, nickel or cobalt base are possible alternatives to ceramic materials. As the operating temperature of metal alloys with a high level of heat resistance is however significantly below the maximum operating temperature of ceramic materials, it is necessary to cool metallic heat shields in hot gas channels.
A heat shield arrangement, in particular for structural elements of gas turbine units, is disclosed in EP 0 224 817 B1. The heat shield arrangement is used to protect a supporting structure against a hot fluid, in particular to protect a hot gas channel wall in gas turbine units. The heat shield arrangement has an internal lining made of heat-resistant material, which generally comprises heat shield elements fixed to the supporting structure. These heat shield elements are disposed next to each other leaving gaps for the passage of cooling fluid and are able to move due to thermal influences. Each of these heat shield elements has a top part and a stem part in the manner of a mushroom. The top part is a flat or three-dimensional, polygonal flat element with straight or curved boundary lines. The stem part connects the central area of the flat element to the supporting structure. The top part is preferably triangular in form, so that an internal lining of almost any geometry can be produced using identical top parts. The top parts and optionally other parts of the heat shield elements are made of a material with a high level of heat resistance, in particular a steel. The supporting structure has holes, through which a cooling fluid, in particular air, can be admitted into an intermediate space between the top part and the supporting structure and from there can be admitted through the gaps for passage of the cooling fluid into a spatial area surrounded by the heat shield elements, for example a combustion chamber of a gas turbine unit. This flow of cooling fluid reduces the penetration of hot gas into the intermediate space.
A metallic lining for a combustion chamber is described in U.S. Pat. No. 5,216,886. This lining comprises a number of cube-shaped hollow elements (cells) disposed next to each other, which are welded or soldered to a common metal plate. The common metal plate has precisely one opening assigned to each cube-shaped cell to admit cooling fluid. The cube-shaped cells are disposed next to each other leaving a gap in between. On every side wall in the vicinity of the common metal plate they have a respective opening for the discharge of cooling fluid. The cooling fluid enters the gap between adjacent cube-shaped cells, flows through said gap and forms a cooling film on a surface of the cells, which is oriented parallel to the metal plate and can be exposed to a hot gas. With the type of wall structure described in U.S. Pat. No. 5,216,886 an open cooling system is defined, in which cooling air passes via a wall structure through the cells into the inside of the combustion chamber. The cooling air is then lost for further cooling purposes.
A wall, in particular for gas turbine units, having cooling fluid channels, is described in DE 35 42 532 A1. In the case of gas turbine units the wall is preferably disposed between a hot space and a cooling fluid space. It is joined together from individual wall elements, each of the wall elements being a plate-type body made from material with a high level of heat resistance. Each plate-type body has parallel cooling channels distributed over its base surface, with one end of said cooling channels communicating with a cooling fluid space and the other end with the hot space. The cooling fluid admitted into the hot space and guided by the cooling fluid channels forms a cooling fluid film on the surface of the wall element facing the hot space and/or adjacent wall elements.
A cooling system for cooling a combustion chamber wall is shown in GB-A-849255. The combustion chamber wall is formed by wall elements. Each wall element has a hot gas wall with an outside that can be subject to the action of hot gas and an inside. Nozzles are disposed at right angles to the inside. Cooling fluid in the form of a concentrated flow is discharged from these nozzles and strikes the inside. This cools the hot gas wall. The cooling fluid is collected in a collection chamber and removed from the collection chamber.
To summarize, all these heat shield arrangements, in particular those for gas turbine combustion chambers, are based on the principle that compressor air is used both as the cooling medium for the combustion chamber and its lining and as sealing air. The cooling and sealing air enters the combustion chamber, without having been involved in combustion. This cold air mixes with the hot gas. This causes the temperature at the combustion chamber exit to drop. As a result the output of the gas turbine drops as does the efficiency of the thermodynamic process. This can be compensated for to some extent by setting a higher flame temperature. However this then gives rise to material problems and higher emission values have to be accepted. Another disadvantage of the specified arrangements is that the admission of a not insignificant mass flow of cooling fluid into the combustion chamber causes pressure losses in the air supplied to the burner.
To prevent coolant blowing out into the combustion chamber, complex systems are known with pressurized cooling fluid control, in which the cooling fluid is guided in a closed circuit with a supply system and a return system. Such closed cooling concepts with pressurized cooling fluid control are described for example in WO 98/13645 A1, EP 0 928 396 B1 and EP 1 005 620 B1.